A study of the nitrogen stable isotope dynamics of phytoplankton in a simple natural ecosystem

We investigated fluctuations in the nitrogen stable isotope ratio (delta(15)N) of Chlamydomonas acidophila (Chlorophyceae) by short-interval sampling during a bloom that occurred from 11 May to 29 July 2001 in Lake Katanuma, Japan. In this lake, C, acidophila uses NH4+ as the primary inorganic nitrogenous nutrient, since only trace amounts of other inorganic nitrogenous compounds are present. The delta(15)N values obtained for C. acidophila ranged from -7.8 to -3.4parts per thousand. during the study period. The lowest values occurred during periods characterized by high concentrations of NH4+ ( > 15 mumol l(-1)), and our results showed that delta(15)N values for C. acidophila were negatively correlated with concentrations of NH4+. The results suggest that this nutrient plays a major role in the variation of the delta(15)N value for this species. The nitrogen isotope fraction factor (epsilon) of C. acidophila was much lower than that of many NH4+-based primary producers from other eutrophic environments.